Bacterial residues in coprolite of herbivorous dinosaurs: Role of bacteriain mineralization of feces

The Cretaceous Two Medicine Formation of northwestern Montana has yielded b locky. calcareous coprolites that contain abundant fragments of conifer woo d and were produced by large herbivorous dinosaurs. The coprolites are gene rally dark gray to black in color due to a dark substance confined chiefly within what originally were the capillaries of tracheid and ray cells of xy lem. This substance is a kerogen that consists in part of thin-walled vesic les 0.1-1.3 gm in diameter. Pyrolysis products of this kerogen are diagnost ic of a bacterial origin with a possible contribution from terrestrial plan ts. The vesicular component is interpreted as the residue of bacterial cell s, whereas a second filamentous component, closely associated with the vesi cles, may be the residue of an extracellular binding material, such as glyc ocalyx. At least two episodes of calcification of the coprolite are recognized by m anganous cathodoluminescence. The earlier of these infilled the capillary c hannels of the conifer fragments. Wood cell walls, voids, cracks, and small burrows were filled during the later episode. Microprobe data confirm thes e results and show that phosphate is sequestered in the capillaries. These observations suggest that bacteria within the capillaries induced initial m ineralization of the coprolite, and, in so doing, created barriers that pro tected organic residues from subsequent destruction. Early onset of mineral ization is consistent with the degree of preservation of woody xylem found in the coprolites.